1. Field of Industrial Application
The present invention relates to a cathode ray tube used in a TV set or a monitor provided with OA equipment, and more particularly to a method for reducing a residual spot caused when switching off a main power supply.
2. Description of the Related Art
When a main power supply is turned off in a cathode ray tube of a TV set or a monitor device, the so-called "residual spot phenomenon" takes place. This phenomenon keeps a display image converted on the center of the display and the center of the display luminous for a short time. This "residual spot phenomenon" presents the image-displaying state on the screen though the main power supply is turned off. Hence, this phenomenon disadvantageously gives a user an uneasy feeling. Further, the "residual spot phenomenon" takes place at a constant point on the display, so that the corresponding fluorescent layer to the point is burned and thus degraded in display characteristic.
In order to overcome this phenomenon, the conventional cathode ray tube has been arranged to cut off electrons emitted from a cathode before a deflection rises and to lower a second grid voltage to 0 V for several tens msec when the main power supply is switched off. The cathode ray tube serves to suppress concentration of the emitted electrons by this function and avoid the "residual spot phenomenon" caused at a certain spot on the display.
The cathode ray tube is arranged so that electrons e.sup.- fired by an electron gun 10 shown in FIG. 1 are controlled by a plurality of grid electrodes and formed on a focal point of the fluorescent screen of the display tube (not shown) for making the screen of the display tube luminous. FIG. 1 shows the first grid electrode 11 to the third grid electrode 13 but does not disclose the other grid electrodes, a convergence electrode for concentrating the electron beam, and the like.
The electron gun 10 provides a cathode housing 14 having an electron emission hole 14a in which a heater 15 serving as a thermal source is built. The cathode housing 14 is made of ceramic, for example. The cathode housing 14 contains cathode-shaped oxide (not shown) coated thereon. This oxide coat serves to generate electrons e.sup.- when the oxide coat is heated by the heater 15. The first grid electrode 11 to the third gird electrode 13 are located in the described order as opposed to the cathode housing 14. The first grid electrode 11 is integrated with the cathode housing 14 for simplifying the overall structure. The second and the third grid electrodes 12 and 13 compose a pre-focus lens.
When the power supply is turned on, the electron gun 10 causes the oxide heated by the heater 15 to fire the electrons e.sup.-. Those electrons e.sup.- are fired from the electron emission hole 14a of the cathode housing 14 and sequentially pass the first grid electrode 11 to the third grid electrode 13, the other grid electrodes, and the convergence electrode, through which the electrons are gradually accelerated and converged into an electron beam. The electron beam is pulled by a high voltage and reaches the fluorescent plane on which a focal point is formed.
By the way, the further main cause of the "residual spot phenomenon" is thermal electrons e.sup.- generated from barium adhering to the grid and evaporated from the oxide composing the cathode body formed on the surface of the cathode. The thermal electrons e.sup.- emitted from the barium adhering to the grid are absorbed by an anode voltage whose rise is delayed by the deflection current when the main power supply is turned off and then are focused on the center of the display, which leads to appearance of the "residual spot phenomenon".
That is, in the electron gun 10, the evaporation of barium Ba from the oxide is prompted when the cathode housing 14 is heated up. As shown in FIG. 1, the barium Ba adheres to the electron emission hole 11a of the first grid electrode 11 and the electron emission hole 12a of the second grid electrode 12. The barium Ba reacts with oxide gas in the cathode ray tube, resulting in generating barium oxide (BaO). The barium oxide is more likely to emit the thermal electrons e.sup.-.
In the electron gun 10, when the power supply is turned on, by heating the first grid electrode 11 and the second grid electrode 12 by virtue of the radiation heat of the heater 15, the thermal electrons e.sup.- are emitted from the oxide barium adhering to the grids. The emission of these thermal electrons e.sup.- from the barium oxide continues, because the first grid electrode 11 and the second grid electrode 12 are kept at high temperature even after the main power supply is turned off.
On the other hand, a high anode voltage HV is applied onto the cathode ray tube so as to guide the electron beam onto the plane of the display screen. When the main power supply is turned off, the rise of the anode voltage HV is later than the deflection current applied to the convergence electrode, because 27 kV is applied to the third gird electrode 13. Hence, when the main power supply is turned off, the thermal electrons e.sup.- are pulled by the anode voltage HV and then are focused on the center of the display, which leads to the appearance of the "residual spot phenomenon".
FIG. 2 is a view showing the "residual spot phenomenon" appearing in the case of gradually lowering the anode voltage HV applied onto the third grid electrode 13 when the first and the second grid electrodes 11 and 12 are both made zero. FIG. 2A shows a spot formed when the anode voltage HV is 23 KV. FIG. ZB shows a spot formed when the anode voltage HV is 22 KV. FIG. 2C shows a spot formed when the anode voltage HV is 20 kV. FIG. 2D shows a spot formed when the anode voltage HV is 19 kV. FIG. 2E shows a spot formed when the anode voltage HV is 18 kV. In the cathode ray tube, in general, the spot formed as shown in FIG. 2C appears. This spot form depends on the timing when the deflecting effect on the electron beam is cut, and on the value of the anode voltage HV on the timing.
As described above, in the cathode ray tube, the thermal electrons e.sup.- are emitted from the barium Ba adhering to the first and the second grid electrodes 11 and 12. Those thermal electrons e.sup.- are pulled onto the display while the application of the anode voltage is kept independently of the cut-off of the cathode. This results in bringing about the "residual spot phenomenon". Hence, this phenomenon cannot be overcome by the foregoing measures. Further, this "residual spot phenomenon" also has a problem that the luminance abruptly rises when the grid voltage applied onto the second grid electrode 12 comes closer to 0 V or some.